The present invention relates to a cryogenic container for storing a cryogen.
A conventional container is disclosed in, e.g., a catalogue for the liquefied helium gas container type RS50 made by L'AIR LIQUIDE in France and sold in Japan through TEIKOKU SANSO Co. Ltd. the present name of which has changed to TEISAN Co. Ltd. Such a container, as shown in FIG. 1, has an inner tank 1 for storing a cryogen 4 therein, an outer tank 2 for housing the inner tank 1 therein and a connecting pipe 3 called a neck tube for supporting the weight of the inner tank 1 and cryogen 4 with respect to the outer tank 2. Space 9 formed between the inner and outer tanks 1 and 2 is vacuum. Shields for radiation heat are disposed in this space in some kind of container.
In the container mentioned above, a combination of the inner and outer tanks 1 and 2 forms a vacuum bottle and the low temperature of the inner tank 1 is kept by the vacuum space 9 with respect to the outer tank 2. The cryogen 4 stored within the inner tank 1 can be taken out through the connecting pipe 3 to the outside of the container. The connecting pipe 3 is constituted by a thermal conductor so that the pipe 3 conducts heat from the outer tank 2 at room temperature to the inner tank 1 at a cryogenic temperature. Although the connecting pipe 3 is designed so as to reduce the heat conducting cross sectional area thereof and increase the length of the heat conducting path of the pipe, there are some restrictions in which the pipe must support the weight of the inner tank 1 and cryogen 4 must be disposed in the limited space 9 between the inner and outer tanks 1 and 2. Due to such restrictions, it is difficult for the connecting pipe 3 to sufficiently meet the above-mentioned requirements about heat conducting cross sectional area and length of heat conducting path.
Since the connecting pipe 3 must mechanically support the weight of the inner tank 1 and cryogen 4 with respect to the outer tank 2 and since the heat conduction through the connecting pipe 3 must be limited as much as possible, the wall thickness or the transverse cross sectional area of the connecting pipe 3 has been determined to provide just enough strength to support the weight of the inner tank 2 and cryogen 4. However, while the strength of material generally increases as the temperature of the material lowers and the inner end of the connecting pipe 3 which is at low temperature is stronger than the outer end of the pipe at room temperature, the conventional connecting pipe 3 is made of a tube of a constant diameter and a constant wall thickness without taking into consideration such changes of material strength according to temperture. Therefore, the end of the pipe near the low temperature inner tank has a transverse cross sectional area which is unnecessarily large in relation to its mechanical strength. This unnecessarily large cross sectional area allows excess heat conduction therethrough.
To overcome the problems mentioned above, an object of the present invention is to provide a cryogenic container in which a connecting pipe can withstand the weight of an inner tank and cryogen and in which the heat conduction through the connecting pipe is reduced.
With the above object in view, the present invention resides in a cryogenic container comprising inner tank means for storing a cryogen therein, outer tank means for housing said inner tank means therein in a thermally insulating relationship, and pipe means connected between said outer and inner tank means for connecting said inner tank means to the outside of said outer tank means, said pipe means supporting said inner tank means away from said outer tank means and having a wall thickness which is greater at the portion near the outer tank side than that near the inner tank side.